Fracture behavior of irradiation induced nanocrystalline UO₂ studied by in-situ mechanical testing in transmission electron microscopy

Uranium Dioxide (UO₂) is widely used as a fuel in current light water reactors (LWRs). Upon accumulation of radiation damage, LWR UO₂ fuel pellets start to develop a different microstructure at the pellet periphery when fuel burnup exceeds 45–50 GWd/tHM. The resulting porous, nanocrystalline microstructure is one of the most prominent microstructural changes occurring in such fuel. Its fracture mechanisms, which causes fuel fine fragmentation, could impact safety limits when the cladding breaches. Direct measurements of these properties are challenging, therefore a surrogate obtained via ion irradiation can be used. In this study, multiple microcantilevers were fabricated by focused ion beam from both fresh UO₂ and UO₂ irradiated with 84 MeV Xe²⁶⁺ ions to a peak dose of 1357 displacements per atom (dpa). The irradiation produced a pseudo high burnup structure approximately 2 µm below the surface. In-situ nano-mechanical bending tests were conducted to investigate the fracture behavior and the effect of the surrogate UO₂ high burnup structure on local fracture properties. Fresh UO₂ fuel was observed to fracture in transgranular mode without nucleation or movement of dislocations. However, the Xe-irradiated nanocrystalline microcantilevers fractured along the grain boundaries, with no influence from the pre-existing micro-cracks in the microcantilever. Fracture toughness for this type of surrogate high burnup UO₂ structure is reported for the first time in literature. Both the fracture stress and toughness show degradation for UO₂ as a result of Xe-irradiation.